Shock absorbing type steering shaft and method for manufacturing the same

ABSTRACT

A shock absorbing type steering shaft comprising a tubular outer shaft having a female serration and a solid inner shaft having a male serration. After the outer diameter surface of the outer shaft is pressed inwardly in the radial direction to plastically deform an inner portion thereof, the inner shaft is inserted into the outer shaft so that the inner shaft is pressure-contacted and fixed to the plastically deformed portion of the outer shaft.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a shock absorbing type steeringshaft which is capable of contracting in the axial direction uponcollision of the car owing to the impact of the collision, and a methodof manufacturing such steering shaft. Particularly, the presentinvention relates to a shock absorbing type steering shaft capable ofstabilizing a collapse load which acts at the time of collision of thecar even when the inner shaft of the car is a solid shaft so as toreduce the manufacturing cost, as well as a method of manufacturing suchsteering shaft.

[0003] 2. Related Background Art

[0004] In a steering apparatus for a car, there is provided a shockabsorbing type steering shaft which can be contracted in the axialdirection upon collision, by the impact of the collision.

[0005] An example of a method for manufacturing such shock absorbingtype steering shaft is disclosed in Japanese Patent ApplicationLaid-Open No. 52-25330, in which a solid inner shaft having anon-circular cross section in a substantially oval form is inserted intoa tubular outer shaft having a non-circular cross section in asubstantially oval form, the outer shaft is pressed by a pressing memberinwardly from the outer side thereof in the radial direction toplastically deform the both shafts locally, and then both the shafts arepressure-fixed to each other elastically by the portions thusplastically deformed. In this manner, while a relative movement betweenthe both shafts in the axial direction is prevented normally, theplastically deformed portions are caused to collapse at the time ofcollision of the car, whereby the inner shaft is moved into the outershaft to contract the entire length of the steering shaft, therebyabsorbing the shock.

[0006] However, according to the manufacturing method disclosed above,since the plastic deforming is conducted in the state that the solidinner shaft is inserted in the outer shaft, an amount of plasticdeformation of the outer shaft is not sufficient, so that there is apossibility of backlash in the pressure contact and fixation of both theshafts.

[0007] In Japanese Patent Application Laid-Open No. 1-58373, in thestate that a jig is inserted in a tubular outer shaft having a circularcross section, the outer shaft is pressed inwardly in the radialdirection by the pressing member from the outer side thereof, so thatthe outer shaft is plastically deformed locally to have a non-circularcross section. Subsequently, the jig is removed, and the solid innershaft having a circular cross section is inserted into the outer shaft,whereby the inner shaft is pressure-fixed to the portion of the outershaft which has been plastically deformed to the noncircular crosssection.

[0008] However, according to the manufacturing method disclosed above,it is required to once insert the jig into the outer shaft and, afterthe plastic processing, to remove the jig. Thus, the manufacturingprocess becomes complicated for such steps of inserting and removing thejig and, as a result, the manufacturing cost is increased.

[0009] Further, in Japanese Patent Application Laid-Open No. 9-272447,an inward protrusion is formed in the radial direction on a tubularouter shaft having a circular cross section with a female serration, anannular recess from which a serration has been removed is formed on theouter peripheral surface of a solid inner shaft having a circular crosssection with a male serration, and the annular recess of the inner shaftis caused to engage with the inward protrusion in the radial directionof this outer shaft in such a manner that the both shafts arepressure-contacted and fixed with pressure to each other.

[0010] However, according to the manufacturing method disclosed above,it is required to remove a part of the serration from the male serrationformed on the outer peripheral surface of the inner shaft and to formthe annular recess, which may result in an increase in the manufacturingcost.

[0011] Further, according to Japanese Patent Application Laid-Open No.10-181615 (which has been converted from an application for a JapaneseUtility Model Registration filed in 1992), the tip end of a tubularouter shaft having a circular cross section and the tip end of a solidinner shaft having a circular cross section are plastically deformed tohave non-circular (flat or elliptic) cross sections, respectively. Onthe other hand, inner portions other than the tip ends of the bothshafts are maintained to have circular cross sections. Then, the innershaft is inserted into the outer shaft, and the portion having thenon-circular cross section of the outer shaft is stronglypressure-contacted and fixed to the portion having the circular crosssection of the inner shaft and the portion having the circular crosssection of the outer shaft is strongly pressure-contacted and fixed tothe portion having the non-circular cross section of the inner shaft,respectively, whereby the both shafts are pressure-contacted and fixedto each other.

[0012] However, according to the manufacturing method disclosed above,the plastic deforming of the tip end of the outer shaft and the plasticdeforming of the tip end of the inner shaft are conducted separately, sothat the manufacturing process becomes complicated, which may bringabout an increase in the manufacturing cost.

[0013] Further, in Japanese Patent Application Laid-Open No. 8-91230,like in Japanese Patent Application Laid-Open No. 10-181615, the tip endof the outer shaft and the tip end of the inner shaft are plasticallydeformed to have non-circular cross sections. However, as a step of thisplastic deforming, the tip end of the inner shaft is inserted in the tipend of the outer shaft to be maintained in an overlapping state, andthese both overlapping tip ends are pressed inwardly in the radialdirection by the pressing members to be plastically deformed to havenon-circular cross sections. Thus, the both tip ends are plasticallydeformed at a time to simplify the manufacturing process.

[0014] However, according to the manufacturing method disclosed above,the inner shaft is required to be hollow and, when the inner shaft issolid, even if the tip end of the solid inner shaft is inserted into thetip end of the tubular outer shaft and the both overlapping tip ends arepressed inwardly in the radial direction by the pressing members, theplastic deforming can not be conducted satisfactorily with respect tothe solid inner shaft.

[0015] Further, according to Japanese Patent Application Laid-Open No.10-147245, like in Japanese Patent Application Laid-Open No. 8-91230mentioned above, the both tip ends are plastically deformed by thepressing members at a time in the state that the both tip ends of theouter shaft and the inner shaft overlap each other. It is furtherarranged to adjust the collapse load by adjusting the press load of thepressing members.

[0016] However, according to the manufacturing method disclosed above,like in Japanese Patent Application Laid-Open No. 8-91230, when theinner shaft is solid, the plastic deforming can not be conductedsatisfactorily.

[0017] Incidentally, when the inner shaft is a solid shaft, it iscomparatively difficult to plastically deform the both tip ends at atime in the state that the tip ends of the both shafts overlap eachother, like Japanese Patent Application Laid-Open No. 8-91230. However,a part of the above Japanese Patent Application Laid-Open No. 8-91230discloses a method for pressure-contacting and fixing the solid innershaft and the tubular outer shaft to each other, as shown in FIG. 19.

[0018] A bottomed hole 103 is formed at the tip end 101 a of the solidinner shaft 101 having a circular cross section with a male serration102, and the tip end 101 a of this inner shaft 101 is fitted in the tipend 104 a of the tubular outer shaft 104 having a circular cross sectionwith a female serration 105. These both overlapping tip ends 101 a and104 a are pressed by the pressing members 106 and 106 from the upper andlower directions thereof so that the both tip ends 101 a and 104 a areplastically deformed to respectively have noncircular (flat or elliptic)cross sections, meanwhile the inner portions other than the tip ends ofthe both shafts 101 and 104 are maintained to have the circular crosssections.

[0019] After that, the pressing members 106 and 106 are removed and theinner shaft 101 is further inserted into the outer shaft 104, the tipend 101 a of the non-circular cross section of the inner shaft 101 isstrongly pressure-contacted and fixed to the inner portion of thecircular cross section of the outer shaft 104, and the tip end 104 a ofthe non-circular cross section of the outer shaft 104 is stronglypressure-contacted and fixed to the inner portion of the circular crosssection of the inner shaft 101.

[0020] However, the drilling and processing works are required to formthe bottomed hole 103 on the tip end 101 a of the solid inner shaft 101,which may bring about an increase in the manufacturing cost.

[0021] While the outer shaft 104 is obtained by plastically deforming atubular shaft, the inner shaft 101 is obtained by plastically deforminga solid shaft on which the bottomed hole 103 is drilled. Thus, the levelof the plastic deforming is not always the same for the both shafts, andone of the shafts may be deformed excessively. As a result, the force ofpressure-contact and fixation is different between the both shafts, sothat it becomes difficult to stabilize the collapse load to act uponcollision of the car.

SUMMARY OF THE INVENTION

[0022] The present invention has been conceived taking the circumstancesmentioned above into consideration, and an object of the invention is toprovide a shock absorbing type steering shaft which can stabilize acollapse load acting upon collision of the car and can reduce themanufacturing cost, as well as a method for manufacturing such steeringshaft.

[0023] In order to achieve the above object, there is provided accordingto the present invention, a shock absorbing type steering shaftcomprising a tubular outer shaft having a female serration and a solidinner shaft having a male serration, characterized in that:

[0024] after the outer diameter surface of the outer shaft is pressedinwardly in the radial direction to plastically deform an axially innerportion thereof, the inner shaft is inserted into the outer shaft sothat the inner shaft is pressure-contacted and fixed to the plasticallydeformed portion of the outer shaft.

[0025] In order to achieve the above object, there is also providedaccording to the present invention a method for manufacturing a shockabsorbing type steering shaft for fitting and fixing a solid inner shafthaving a male serration in a tubular outer shaft having a femaleserration, comprising the steps of:

[0026] inserting the tip end of the inner shaft into the tip end of theouter shaft to hold the same in a state that these tip ends areoverlapping each other;

[0027] pressing the outer shaft at which the both tip ends are notoverlapping each other inwardly in the radial direction by means of apressing member to plastically deform an axially inner portion thereof;and

[0028] removing this pressing member to further insert the inner shaftinto the outer shaft, thereby pressure-contacting and fixing the innershaft to the plastically deformed portion of the outer shaft.

[0029] As describe above, according to the present invention, the tipend of the inner shaft is inserted into the tip end of the outer shaftto be maintained in an overlapping manner, the inner portion of theouter shaft is pressed inwardly in the radial direction by the pressingmembers and is plastically deformed, and the inner shaft ispressure-contacted to be fixed to this plastically deformed portion ofthe outer shaft. That is, the outer shaft is plastically deformed whilepreventing the outer tube from being excessively deformed, byplastically deforming the inner portion of the outer shaft in the statethat the tip ends of the both shafts overlap each other slightly.Consequently, it is possible to stably maintain the force of pressurecontact and fixation of the steering shaft in its accomplished state. Itis also possible to stabilize a collapse load which acts at the time ofcollision of the car. In addition, since a drilling step is unnecessary,unlike in the conventional method, the manufacturing cost can bereduced.

[0030] According to another aspect of the present invention, there isprovided a method for manufacturing a shock absorbing type steeringshaft in which a solid inner shaft having a male serration ispressure-fitted in and fixed to a tubular outer shaft having the femaleserration, comprising the steps of:

[0031] pressing and deforming a predetermined portion of the tip end ofthe outer shaft from the outside thereof in the radial direction to formthe female serration portion; and

[0032] pressing and fitting the male serration portion of the innershaft in this pressed and deformed portion to pressure-contact and fixthe inner shaft thereto. There is also provided a shock absorbing typesteering shaft which is manufactured by such method.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a schematic cross sectional view for showing a step of amethod for manufacturing a shock absorbing type steering shaft accordingto the first embodiment of the present invention.

[0034]FIGS. 2A to 2D are schematic cross sectional views, in which FIG.2A shows a cross section taken along the line 11-11 in FIG. 1, and FIGS.2B, 2C and 2D, respectively, show steps of the plastic deformingaccording to the variations.

[0035]FIG. 3 is a partial cross sectional view of the shock absorbingtype steering shaft according to the first embodiment of the presentinvention.

[0036]FIG. 4 is a graph for showing a relationship between a press-inforce and a stroke when the large diameter portion of the inner shaft isinserted into the small diameter portion of the outer shaft to bebrought into pressure contact and fixation, in the shock absorbing typesteering shaft according to the first embodiment of the presentinvention.

[0037]FIG. 5 is a schematic cross sectional view for showing a step of amethod for manufacturing a shock absorbing type steering shaft accordingto the second embodiment of the present invention.

[0038]FIG. 6 is a schematic view of a steering mechanism in which theshock absorbing type steering shaft according to an embodiment of thepresent invention is assembled.

[0039]FIG. 7 is a schematic cross sectional view for showing a step of amethod for manufacturing a shock absorbing type steering shaft accordingto the third embodiment of the present invention.

[0040]FIG. 8 is a schematic cross sectional view for showing a step of amethod for manufacturing a shock absorbing type steering shaft accordingto the fourth embodiment of the present invention.

[0041]FIG. 9 is a schematic cross sectional view for showing a step of amethod for manufacturing a shock absorbing type steering shaft accordingto a variation of the fourth embodiment of the present invention.

[0042]FIG. 10 is a schematic cross sectional view for showing a step ofa method for manufacturing a shock absorbing type steering shaftaccording to the fifth embodiment of the present invention.

[0043]FIGS. 11A to 11D are schematic cross sectional views for showingfour pressing portions used in a method for manufacturing a shockabsorbing type steering shaft according to the first variation of thefifth embodiment of the present invention.

[0044]FIGS. 12A, 12B and FIGS. 13A, 13B are schematic cross sectionalviews for showing two pairs of pressing portions used in a method formanufacturing a shock absorbing type steering shaft according to thesecond and third variations of the fifth embodiment of the presentinvention.

[0045]FIG. 14 is a cross sectional view for schematically showing a stepof a method for manufacturing a shock absorbing type steering shaftaccording to the sixth embodiment of the present invention.

[0046]FIG. 15 is a schematic cross sectional view for showing a step ofa method for manufacturing a shock absorbing type steering shaftaccording to the seventh embodiment of the present invention.

[0047]FIG. 16 is a schematic cross sectional view for showing a step ofa method for manufacturing a shock absorbing type steering shaftaccording to the eighth embodiment of the present invention.

[0048]FIG. 17 is a schematic cross sectional view for showing a step ofa method for manufacturing a shock absorbing type steering shaftaccording to the twelfth embodiment of the present invention.

[0049]FIG. 18 is a schematic cross sectional view for showing a step ofa method for manufacturing the shock absorbing type steering shaftaccording to the twelfth embodiment of the present invention.

[0050]FIG. 19 is a schematic cross sectional view for showing a step ofa method for manufacturing a shock absorbing type steering shaftaccording to the prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051]FIG. 6 is a schematic view of a steering mechanism for a steeringapparatus of a car, in which a shock absorbing type steering shaftaccording to an embodiment of the present invention, which will bedescribed later, is assembled. As shown in FIG. 6, a steering wheel 2 isfixed to the upper end of a steering shaft 1. A steering column 3 isfixed to the lower surface of an instrument panel 6 by means of upperand lower brackets 4 and 5. The first steering shaft 1 is insertedthrough inside this steering column 3 to be rotatable. A portion whichis positioned at a lower end of the first steering shaft and isprotruding from the lower end opening of the steering column 3 iscoupled to the upper end portion of a second steering shaft 8 through afirst universal joint 7. The lower end portion of this second steeringshaft 8 is coupled to a third steering shaft 110 which leads to asteering gear (not shown) through a second universal joint 9.

[0052] With the constitution described above, a movement of the steeringshaft 2 is transmitted to the steering gear through the first steeringshaft 1 which is inserted through the steering column 3, the firstuniversal joint 7, the second steering shaft 8, the second universaljoint 9, and the third steering shaft 110. This steering gear gives asteering angle to the wheels in response to the movement of the steeringwheel 2.

[0053] In this steering mechanism, either one or both of the steeringshafts 1 and 8 have the forms in the following embodiment of the presentinvention.

[0054] Description will be made below on a shock absorbing type steeringshaft and a method for manufacturing the same according to an embodimentof the present invention, with reference to the drawings.

[0055]FIG. 1 is a schematic cross sectional view for showing a step ofthe method for manufacturing a shock absorbing type steering shaftaccording to a first embodiment of the present invention. FIG. 2A is across sectional view of the steering shaft, taken along the line II-ITin FIG. 1, for showing a step of plastic deforming process. FIGS. 2B,2C, and 2D are schematic cross sections for respectively showing stepsof plastic deforming process according to variations of the embodiment.

[0056] As shown in FIG. 1, a large diameter portion 11 having a maleserration 12 is formed on a solid inner shaft 10 having a circular crosssection, while a small diameter portion 15 having a female serration 16is formed on a tubular outer shaft 14 having a circular cross section.When a plastically deforming process which will be described later isconducted, the tip end 13 of the inner shaft 10 is inserted into the tipend 17 of the outer shaft 14 to be maintained in an overlapping manner.

[0057] A pair of pressing members 18 and 19 are disposed for plasticallydeforming the small diameter portion 15 of the outer shaft 14. Thesepressing members 18 and 19 are respectively provided with tip end sidepressing portions 20 and 21 for plastically deforming the tip end sidesof the small diameter portion 15 and rear side pressing portions 22 and23 for plastically deforming the rear sides of the small diameterportion 15.

[0058] As shown in FIG. 2A, arcuate pressing surfaces 24 a and 24 b areformed on the respective pressing portions 20 and 21; and 22 and 23 ofthe pressing members 18 and 19.

[0059] Note that as variations of the pressing members 18 and 19, thepressing portions 20 to 23 may have deep grooves 25 a and 25 b eachhaving a U-shaped cross section, as shown in FIG. 2B, so that the fouredges of these deep grooves 25 a and 25 b may be used for pressing.Also, as shown in FIG. 2C, the pressing portions 20 to 23 may be formedto be entirely flat. Further, as shown in FIG. 2D, the pressing members21 and 23 on one side may have a deep groove 26 having a V-shaped crosssection, and three positions disposed at equal intervals in thecircumferential direction of this deep groove may be used for pressing.Note that these pressing portions 20 to 23 may have various forms shownin FIGS. 2A to 2D in a mutually combined manner, and may have otherforms not shown in the drawings.

[0060] According to the above constitution, when the shock absorbingtype steering shaft is to be manufactured, first the tip end 13 of thelarge diameter portion 11 of the inner shaft 10 is inserted in the tipend 17 of the small diameter portion 15 of the outer shaft 14, so as tobe maintained in an overlapping manner.

[0061] Next, a part of the small diameter portion 15 of the outer shaft14 in which the both tip ends 13 and 17 do not overlap each other ispressed inwardly in the radial direction by means of the pair ofpressing members 18 and 19 and is plastically deformed.

[0062] In this manner, the part of the small diameter portion 15 whichis plastically deformed by means of the tip end side pressing portions20 and 21 and the part of the small diameter portion 15 which isplastically deformed by the rear side pressing portions 22 and 23 aredeformed respectively to have non-circular (flat, elliptic, or the like)cross sections, meanwhile a portion in the middle of these deformedportions is maintained to have a substantially circular cross section.

[0063] Subsequently, these pressing members 18 and 19 are removed, andthen the large diameter portion 11 of the inner shaft 10 is furtherinserted (pressure-inserted) in the small diameter portion 15 of theouter shaft 14, so that the large diameter portion 11 is brought intopressure-contact to be fixed to the two parts of the small diameterportion 15 which are plastically deformed to have non-circular crosssections to provide the condition shown in FIG. 3.

[0064] Next, FIG. 4 shows a relationship between a stroke and a press-inforce for inserting the large diameter portion 11 of the inner shaft 10into the small diameter portion 15 of the outer shaft 14 to bepressure-contacted and fixed thereto.

[0065] Referring to FIG. 4, the area “A” indicated by the virtual lineshows a time period from the start of insertion of the inner shaft 10into the outer shaft 14 to the end of the insertion (completion of theassembling) Meanwhile, the area “B” indicated by the solid line showsthe time of collision of the car after the attachment of the shaft tothe car.

[0066] In the area “A” for showing the assembling process, it may besummarized that the press-in force takes a lower peak value (indicatedby the mark a) when the large diameter portion 12 is inserted into thepart of the small diameter portion 15 which is plastically deformed bymeans of the tip end side pressing portions 20 and 21, while thepress-in force takes a higher peak value (indicated by the mark c) whenthe large diameter portion 12 is inserted into the part of the smalldiameter portion 15 plastically deformed by means of the rear sidepressing portions 22 and 23.

[0067] The mark b indicates a state in which, after the large diameter12 is inserted into the part of the small diameter portion 15 which isplastically deformed by means of the tip end side pressing portions 20and 21, the large diameter portion 12 is fitted in this part of thesmall diameter portion 15 along a predetermined width. In this state,the press-in force indicate a substantially the same value, loweringfrom the lower peak value.

[0068] Further, the mark d indicates a state of completion of theassembling in which the large diameter portion 12 is fitted in the twoparts of the small diameter portion 15 which have been plasticallydeformed by means of the tip end side pressing portions 20 and 21 andthe rear side pressing portions 22 and 23 along a predetermined length.In this state of completion of the assembling, the press-in forceindicates a substantially stabilized fixed value, lowering from thehigher peak value. This press-in force value acts as a collapse load.

[0069] On the other hand, in the area “B” which shows the state afterthe attachment of the shaft to the car, the collapse load for acting atthe time of secondary collision of the car takes a substantiallystabilized fixed value indicated by the mark d. More specifically, whenthe impact load at the time of collision of the car exceeds the press-inforce value indicated by the mark d, the both shafts 10 and 14 collapseand are contracted at the two pressure-contacted and fixed parts.According to the present embodiment, this collapse value (the press-inforce value indicated by the mark d) is very stable so that scoringphenomenon is difficult to occur.

[0070] The press-in force value (the collapse load value) indicated bythe mark d or the press-in force value indicated by the mark b can becontrolled in various manners by adjusting the pressing force caused bythe tip end side pressing portions 20 and 21 and the rear side pressingportions 22 and 23.

[0071] Note that the mark e shows the state in which the fitting at theplastically deformed part on the tip end side of the small diameterportion 15 is released after the collapse, and only the fitting at theplastically deformed part on the rear side is maintained.

[0072] As described above, in the present embodiment, the collapse valueis very stable, which is supposedly caused by the fact that the smalldiameter portion 15 of the outer shaft 14 is subjected to plasticdeformation in a state that tip ends 13 and 17 of the both shafts 10 and14 slightly overlap each other so that the plastic deforming can beconducted while preventing an excessive deformation of the outer tube14, whereby the elastic fitting elements can be enhanced (that is,supposedly caused by the fact that two plastically deformed portions ofthe small diameter portion 15 are not excessively deformed so that thereremains a room for further elastic deformation after the plasticdeforming process). As a result, when the both shafts are brought intopressure-contact to be fixed to each other, the force of thepressure-contact and fixation can be stably maintained, so as tostabilize the collapse load at the time of collision of the car.

[0073] Note that since the press-in force value indicated by the mark d(the collapse load value) is very stable, as described above, it ispossible to suppress the lowering of this press-in force value (thecollapse load value) to the minimum even when the steering shaft is tobe re-assembled.

[0074] Also, since the drilling step which was required in the priorart, is unnecessary, it is possible to reduce the manufacturing cost.

[0075] Next, FIG. 5 is a schematic cross sectional view for showing astep of the manufacturing method of a shock absorbing type steeringshaft according to a second embodiment of the present invention.

[0076] In the second embodiment, there are provided a pair of pressingmembers 27 and 28, and only one part of the small diameter portion 15 ofthe outer shaft 14 is subjected to a plastic deforming process. Also inthis case, like in the first embodiment, the small diameter portion 15of the outer shaft 14 is subjected to plastic deformation in a state inwhich the tip ends 13 and 17 of the both shafts 10 and 14 slightlyoverlap each other, so that it is possible to conduct the plasticdeformation while preventing an excessive deformation of the outer tube14, thereby stabilizing the collapse load at the time of secondarycollision.

[0077] Note that the cross sections of these pressing members 27 and 28may be any of those shown in FIGS. 2A-2D, and may be any forms otherthan those.

[0078] The basic concept of the present invention was described above byway of the first and second embodiments. Description will be furthermade specifically on a method of manufacturing a shock absorbing typesteering shaft which is suitable for a practical use.

[0079] In the foregoing embodiments, each corner of the pressing membertakes an angular edge-like form. For this reason, the plasticdeformation of the outer shaft tends to occur comparatively with sudden.Thus, the scoring easily takes place when the inner shaft ispressure-fitted into the outer shaft, and the surface pressure rises andan apparent fitting load rises, so that the pressurization by thepressing members becomes insufficient and the durability is required tobe further enhanced. In addition, even if the apparent press-in load issufficiently high, an elastic interference in the fitting portionbetween the outer shaft and the inner shaft is insufficient. Furtherenhancement of the durability is desired in this point.

[0080] The following embodiment is to provide a specific method ofmanufacturing a shock absorbing type steering shaft which is capable ofsolving the above-mentioned problems, has a more excellent durabilityand is more suitable for the practical use.

[0081]FIG. 7 shows a third embodiment of the present invention which isobtained by improving the above-described first embodiment. In the thirdembodiment, a pair of pressing members 118 and 119 are provided with twosets of integrally formed pressing portions 120 and 122; and 121 and123, which are opposed to each other, respectively. In the thirdembodiment, the corners of each pressing portion are rounded orR-chamfered. Therefore, as described with respect to the firstembodiment, when the outer shaft 14 is to be pressed by the pressingmembers 118 and 119 to be plastically deformed, the form of the portionto be deformed is gradually deformed. As a result, when the tip end 11of the inner shaft 10 is inserted to be fitted in the small diameterportion 15 of the outer shaft, a pressing load is gradually applied onthe inner shaft, thereby giving a sufficient interference.

[0082] Further, if the tip end 113 of the inner shaft 10 is slightlycorner-sloped or R-chamfered, as shown in FIG. 7, the same effect can beobtained.

[0083] Other arrangements of the third embodiment shown in FIG. 7 arethe same as those of the first embodiment shown in FIG. 1.

[0084]FIG. 8 shows a fourth embodiment of the present invention which isobtained by improving the above-described second embodiment. In thefourth embodiment, each of the paired pressing members 118 and 119 isprovided with one pressing portion. The two pressing portionsrespectively provided for the pressing members 118 and 119 are opposedto each other. In the fourth embodiment, the corners of each pressingportion are rounded or R-chamfered. Therefore, as described above withrespect to the second embodiment, when the outer shaft is pressed by thepressing members to be plastically deformed, the form of the portion tobe deformed is gradually deformed. As a result, the inner shaft isinserted and pressure-fitted in the outer shaft, the load is graduallyapplied on the inner shaft, whereby a sufficient interference can beallowed.

[0085] Further, if the tip end 113 of the inner shaft 10 is slightlycorner-sloped or R-chamfered, as shown in FIG. 8, the same effect can beobtained.

[0086] Other arrangements of the fourth embodiment shown in FIG. 8 arethe same as those of the second embodiment shown in FIG. 5.

[0087]FIG. 9 relates to a variation of the fourth embodiment. In thisvariation, the tip end of the inner shaft 10 is chamfered. Otherarrangements of the variation are the same as those of the embodiment ofFIG. 8, and the same effect as that of the third embodiment can beobtained.

[0088]FIG. 10 is a view for explaining the fifth embodiment which isobtained by improving the first embodiment of the present invention. Inthe fifth embodiment, two sets of pressing members 218 a, 218 b; and 219a, 219 b are separately provided at the two positions in the axialdirection. In the fifth embodiment, the forms of the pressing membersmay be any of those shown in FIGS. 11A to 11D singly or in combination,and may be any other form not illustrated. Note that, of the pressingmembers in FIG. 11B, the corners of the pressing portions are rounded.

[0089]FIGS. 12A, 12B and 13A, 13B respectively relate to second andthird variations of the fifth embodiment. In these variations, thepressurizing directions of the two sets of the pressing members 218 a,218 b; and 219 a, 219 b provided at the two positions in the axialdirection differ from each other substantially by 90 degrees.

[0090] According to the fifth embodiment, it is possible to set anoptimal pressing load to be more suitable for the form, the dimensions,the material, the thickness, and the like, of the product, by employingdifferent pressing members in the axial direction. It is also possibleto optimally control an interference ratio or a fitting load.

[0091] According to the first and second variations of the fifthembodiment shown in FIGS. 12 and 13, it is further possible to effectbalance control in the vertical direction and the horizontal directionwith respect to the restraining force in the shaft bending direction.

[0092]FIG. 14 shows a sixth embodiment which is obtained by improvingthe third embodiment. In the sixth embodiment, a pair of pressingmembers 318 and 319 are provided with two integrally formed pressingportions, which are opposed to each other, like in the third embodiment.The corners of each pressing portion are R-chamfered or rounded.

[0093] In the sixth embodiment, the heights of the two pressing portionsare different from each other, and particularly an interference of thedeformed portion on the inner side (in the left part in the drawing) ofthe outer shaft is changed, so as to adjust the press-in load. Accordingto the sixth embodiment, it is possible to desirably adjust the press-inload by thus differentiating the heights of the two pressing portionsfrom each other.

[0094] Other arrangement of the sixth embodiment shown in FIG. 14 arethe same as those of the third embodiment shown in FIG. 7.

[0095]FIG. 15 shows a seventh embodiment of the present invention. Inthe seventh embodiment, a pair of pressing members take the same formsas those in the third embodiment.

[0096] Further, in the seventh embodiment, a force for maintaining theelasticity between the deformed portion and an interference fittingportion of the inner shaft is adjusted by locally changing the thicknessof a portion 415 to be deformed of the outer shaft 14.

[0097]FIG. 16 shows an eighth embodiment of the present invention. Inthe eighth embodiment, a pair of pressing members take the same forms asthose in the fourth embodiment.

[0098] Further, in the eighth embodiment, the thickness of a portion tobe deformed of the outer shaft 14 is changed locally, like in theseventh embodiment, thereby a force for maintaining the elasticitybetween the deformed portion of the inner shaft and an interferencefitted portion of the inner shaft being adjusted.

[0099] Next, a ninth embodiment (not shown) of the present inventionwill be described. In case of the ninth embodiment, portions to bepressure-fitted of the outer shaft and/or the inner shaft are subjectedto a surface treatment. With this surface treatment, a metallic soap, aplating, or the like, is interposed between the pressure-fittedportions, so that the scoring does not easily occur and a coefficient offriction is lowered. Consequently, the same effect as that describedwith respect to the third and fourth embodiments can be obtained.

[0100] A tenth embodiment (not shown) of the present invention will bedescribed. In case of the tenth embodiment, serration portions ofportions to be pressure fitted of the outer shaft and/or the inner shaftare subjected to a plastic processing. With this plastic processing, thesurfaces to be pressure-fitted become plastically processed surfaces, sothat the surface roughness thereof is enhanced and a coefficient offriction which is obtained from the plastically processed surface islowered. As a result, the same effect as that described with respect tothe third and fourth embodiments can be obtained.

[0101] An eleventh embodiment (not shown) of the present invention willbe described in the following. In case of the eleventh embodiment, thehardness of a portion to be pressure-fitted of the inner shaft is set tobe higher than that of the outer shaft by 30 points or more in theVickers hardness. Thus, scoring is hardly generated when the inner shaftis pressure-fitted in the outer shaft, and the fitting load can besuppressed to be comparatively low.

[0102] The pressing members used in the ninth to eleventh embodimentsmay be any of those used in the first to eighth embodiments.

[0103]FIG. 17 shows a twelfth embodiment of the present invention. Inthe twelfth embodiment, unlike in any of the foregoing embodiments, aportion 814 desired to be deformed is locally pressed by pressingmembers 850 to 853 when a female serration portion is formed on theouter shaft 14. The pressurization is released upon completion of theformation of the serration, thereby completing an elliptic serrationportion. It is possible to obtain a shock absorbing type steering shafthaving a sufficient fitting force without backlash, like in any of-t-heforegoing embodiments, by pressure-fitting an inner shaft having a maleserration of a true circular form in this serration portion. Also, asshown in FIG. 18, if a ring 860 for preventing deformation is providedwhen forming a serration, a true circular outer periphery can be formedon a desired portion at which seals or bearings can be provided easilyand securely. Note that in FIG. 17, a reference numeral 800 denotes asupporting table, and the arrow indicates an advancing direction of abroaching tool for serration.

[0104] According to the twelfth embodiment, the arrangement simply isthat the inner shaft is pressure-fitted in the outer shaft, so that theprocessing cost can be reduced.

[0105] Note that the present invention is not limited to the foregoingembodiments, but allows a large number of variations.

[0106] As described above, when the outer shaft and the inner shaft arebrought into pressure-contact and fixed to each other, it is possible tostably maintain this force of pressure-contact and fixation, therebystabilizing the collapse load which acts upon the secondary collision.It is also possible to reduce the manufacturing cost since the drillingstep is unnecessary, unlike in the conventional example.

What is claimed is:
 1. A method for manufacturing a shock absorbing typesteering shaft for fitting and fixing a solid inner shaft having a maleserration in a tubular outer shaft having a female serration,comprising: pressing and deforming a portion or portions of the outershaft from the outside thereof in a radial direction while forming thefemale serration therein; and pressure-fitting the male serration of theinner shaft into the pressed and deformed portion or portions of theouter shaft to contact and fix the inner shaft to the outer shaft.